Method and apparatus for producing hydrophobic silica fine powder

ABSTRACT

Hydrophobic silica fine powder is produced by pyrolyzing a silane compound to form a silica fine powder and hydrophobizing the silica fine powder with an organohalosilane in a fluidization vessel. Hydrophobized silica fine powder which flies out of the fluidization vessel is collected with a cyclone and bag filter which are held at a temperature of 100-500° C. An apparatus for carrying out the process is also provided. Under simple controlled conditions that involve holding the cyclone and bag filter for recovering fugitive silica from the fluidization vessel to temperatures of 100-500° C., the method and apparatus are able to recover essentially 100% of fugitive silica, thus increasing yield of the product and alleviating the burden on waste gas treatment.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and apparatus forproducing hydrophobic silica fine powder which can be used as athickener for coatings, adhesives and synthetic resins, as areinforcement for plastics, and to improve flowability in toners forcopiers.

[0003] 2. Prior Art

[0004] Pyrogenic silica (silicon dioxide) is very fine, having aparticle size of about 5 to 50 nm. Because it is difficult to collect inthis form, it is agglomerated, then collected. The agglomerated silicacontains a high concentration of chlorine, and must therefore bedeacidified. Deacidification is generally carried out in a fluidizationvessel. When agglomerated silica is deacidified, only a small amount ofsilica flies out of the fluidization vessel together with waste gases.However, when the silica is treated with a hydrophobizing agent, due tobreakup of the agglomerate by such treatment, at least several timesmore treated silica flies out of the fluidization vessel together withwaste gases than when agglomerated silica is directly deacidified. Thepresence of such fugitive treated silica in the waste gases leads to anumber of practical obstacles when the waste gases are treated with ascrubber, such as the formation of foam, which cannot be easily removedwith filters.

SUMMARY OF THE INVENTION

[0005] It is therefore an object of the invention to provide a methodand apparatus for producing hydrophobic silica fine powder byhydrophobizing agglomerated silica with an organohalosilane in afluidization vessel, which are designed such that a part of the treatedsilica which flies out of the vessel together with waste gases can bereliably recovered without complicating the apparatus or processcontrol.

[0006] In studies where we installed cyclones and bag filters to recoversilica that had flown out from fluidization vessels and examined thedegree of fly-out based on the amount of silica recovered, we found thefly-out ratio to be 0.3 to 0.5% when conventional pyrogenic silica isdeacidified in a fluidization vessel, and 4 to 15% when such silica isfirst treated with a hydrophobizing agent then deacidified. While theshape of the equipment and the fluidizing conditions also have an effecton the fly-out ratio, this large difference appears to be attributableto the breakup of agglomerates in hydrophobizing treatment, which leadsto easier fly-out than when the silica is subjected only todeacidification. Recovery of the fugitive silica is thus necessary toimprove product yield and alleviate the burden on waste gas treatment.

[0007] However, unreacted organohalosilane (referred to hereinafter as“silane”) hydrophobizing agent present in the waste gases forms a gel oroil due to the condensation of moisture in the waste gases, which canlead to the clogging and obstruction of equipment and lines. Onmeasuring and studying the temperature at various places in the exhaustsystem, we have found that, if the temperature of the equipment andwaste gases is maintained at 100° C. or higher, the moisture present inthe waste gases does not condense and undesirable products such as gelsor oils due to moisture and unreacted silane do not form. In particular,the absence of gel or oil formation on the filter fabric in a bag filterkeeps the filter fabric free of clogging, making it possible to carryout continuous operation.

[0008] The degree of fly-out also varies with the flow conditions. Inhydrophobizing treatment, a high concentration of chlorine is generallypresent in the gas, creating a need for subsequent deacidification.However, it is more effective to carry out hydrophobizing treatment anddeacidification separately, in which case the presence or absence ofmoisture comes to have an effect on flow of the material duringdeacidification. An investigation on the level of water showed us thatmaterial fluidization is poor in the absence of moisture, but that theaddition of even a very small amount of water to the fluidizing gasimproves the flow state and reduces fly-out. Less fly-out makes itpossible to lower the burden on cyclones and especially bag filters.

[0009] We thus discovered that by holding down fly-out and maintainingthe temperature of the cyclone and bag filter at 100° C. or higher,essentially 100% of fugitive silica can be recovered.

[0010] Accordingly, the invention provides a method for producinghydrophobic silica fine powder. A silane compound is pyrolyzed to form asilica fine powder. The silica fine powder is then hydrophobized with anorganohalosilane in a fluidization vessel, giving hydrophobized silicafine powder which is collected. The hydrophobized silica fine powderwhich flies out of the fluidization vessel is collected with a cycloneand bag filter which are held at a temperature of 100 to 500° C.

[0011] In a preferred embodiment, the fluidization vessel includes ahydrophobizing section where the silica fine powder is hydrophobized anda deacidifying section where deacidification is carried out followinghydrophobization. Deacidification is preferably carried out in thedeacidifying section by adding 0.1 to 1 vol % of water to a fluidizinggas.

[0012] The invention also provides an apparatus for producinghydrophobic silica fine powder, which apparatus includes a means forpyrolyzing a silane compound to form silica fine powder, a means foragglomerating the silica fine powder, a first cyclone and a first bagfilter for collecting the agglomerated silica fine powder, afluidization vessel having a hydrophobizing section for hydrophobizingthe collected silica fine powder, and a second cyclone and a second bagfilter for collecting the hydrophobic silica fine powder which flies outof the fluidization vessel. The second cyclone and the second filter caneach be held at a temperature of 100 to 500° C.

[0013] The advantages of the invention are as follows. When silane isflame-hydrolyzed to form silica fine powder, and the silica is thenhydrophobized in a fluidization vessel using a hydrophobizing agent suchas an organohalosilane, the amount of silica that flies out of thevessel into the waste gases is greater than when hydrophobizingtreatment is not carried out. During recovery of the silica in the wastegases, the condensation of moisture in the waste gases convertsunreacted organohalosilane hydrophobizing agent which emerges togetherwith the waste gases into an undesirable gel or oil. In the method andapparatus of the invention, by maintaining the cyclone and bag filterused as the recovery devices at a temperature of at least 100° C., noorganohalosilane gel or oil forms and thus no clogging of lines or bagfilter pores occurs, making continuous operation possible. Moreover, theinventive method and apparatus enable essentially 100% recovery offugitive silica, resulting in a higher product yield. An additionaladvantage is that, even when the waste gases are treated with ascrubber, there is little if any fugitive silica-induced formation offoam, which cannot be easily removed with filters. This greatlyalleviates the burden on waste gas and wastewater treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The objects, features and advantages of the invention will becomemore apparent from the following detailed description, taken inconjunction with the accompanying drawings.

[0015]FIG. 1 is a flow diagram illustrating an embodiment of theinvention.

[0016]FIG. 2 is a flow diagram illustrating Comparative Example 1described below.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The inventive process for producing hydrophobic silica finepowder involves pyrolyzing a silane compound (a halogenated siliconcompound) to form a silicon dioxide fine powder (pyrogenic silica), thentreating the pyrogenic silica in a fluidization vessel with ahydrophobizing agent, more specifically an organohalosilane.

[0018] The pyrogenic silica may be prepared by a known process using ahalogenated silicon compound such as methyl-trichlorosilane. A silicapowder having a BET specific surface area of 50 to 400 m²/g is desirablein terms of flowability and other characteristics.

[0019] After pyrogenic silica is prepared by a known method from ahalogenated silicon compound, it is preferably agglomerated and halogengases such as chlorine are separated off and removed. Thereafter, theagglomerated silica is hydrophobized in a fluidization vessel using anorganohalosilane as the hydrophobizing agent and using also steam and aninert gas. In a preferred embodiment, the fluidization vessel is dividedinto a hydrophobizing section and a deacidifying section.Hydrophobization of the pyrogenic silica is carried out in thehydrophobizing section, followed by deacidification in the deacidifyingsection.

[0020] In the practice of the invention, a part of the hydrophobizedsilica fine powder which flies out of the fluidization vessel (includingboth the hydrophobizing section and the deacidifying section) iscollected with a cyclone and bag filter held at temperatures within arange of 100 to 500° C. The collected powder is returned to thefluidization vessel, and in particular the deacidifying section. In thedeacidifying section, adding 0.1 to 1 vol % of water to the fluidizinggas is preferable for promoting fluidization and deacidification.

[0021] In one preferred embodiment, production and recovery ofhydrophobized silica fine powder is carried out as a continuous processwithin an apparatus that includes a pyrogenic silica-producingoperation. However, this is not an essential feature of the invention.

[0022] Referring to FIG. 1, a preferred embodiment of the invention isdescribed below. Pyrogenic silica is produced according to aconventional process by burning a halogenated silicon compound togetherwith hydrogen and air in a combustion chamber (pyrolyzing means) 1 andagglomerated by an agglomerator (agglomerating means) 2 for subsequentcollection by cyclones 3 and a bag filter 4. Use of the cyclones 3 andbag filter 4 also serves to separate off chlorine and otherhalogen-containing gases that form as by-products in the combustionchamber 1. The separated halogen-containing gases are sent to ascrubber. The agglomerated silica then passes through rotary valves 5and is collected in a hopper 6. Agglomerated silica that has beenretrieved by the bag filter 4 also is recovered in the hopper 6.

[0023] Next, the agglomerated silica passes through a double damper 7,and is delivered by a diaphragm pump 8 to a fluidization vessel 9 forhydrophobization.

[0024] The fluidization vessel 9 is divided into a hydrophobizingsection A and a deacidifying section B. In the apparatus depicted inFIG. 1, the hydrophobizing section A and the deacidifying section Bcommunicate in the lower portion of the fluidization vessel 9. Silicahydrophobization is carried out in hydrophobizing section A, and thehalogen gas such as chlorine which accompanies the silica from thehydrophobizing section A is removed in the deacidifying section B.Alternatively, hydrophobization and deacidification may be carried outin separate devices.

[0025] In the hydrophobizing section A, the silica is fluidized with aninert gas, generally nitrogen (N₂), and is treated with a hydrophobizingagent. In the apparatus shown in FIG. 1, the hydrophobizing agent 10 issent by a pump 11 through a vaporizer 12 and to the fluidization vessel9. The hydrophobizing agent 10 may be mixed with the silica before thesilica enters the fluidization vessel 9. An alternative is to heatfluidizing nitrogen having water entrained thereon, then mix thehydrophobizing agent into the gas stream and introduce the resultingmixture into the fluidization vessel 9.

[0026] The silica is hydrophobized at a temperature of preferably 400 to600° C., and most preferably 450 to 550° C. The flow velocity ispreferably from 1 to 6 cm/s, although a velocity within a range of 1.4to 3 cm/s is especially preferred for achieving a stable fluidized stateand holding down the fly-out of silica. Water is used at this pointbecause it has a beneficial effect on hydrophobizing treatment. Thewater 14 is fed with a pump 15 to the fluidizing inert gas, followingwhich the gas is heated with a heater 13 and introduced to thehydrophobizing section A of the fluidization vessel 9. The amount ofwater used for hydrophobization is preferably 0.1 to 5 parts by weight,and most preferably 0.5 to 3 parts by weight, per 100 parts by weight ofsilica. The hydrophobizing agent is an organohalosilane, and mostpreferably dimethyldichlorosilane.

[0027] In the deacidifying section B, the silica is fluidized with aninert gas, typically nitrogen, and subjected to deacidification. Wateris typically added to the fluidizing gas so that deacidification can becarried out in a water-containing atmosphere. Preferably, as shown inFIG. 1, the water 16 is added to the fluidizing gas with a pump 17,following which the gas is heated with a heater 13 and introduced to thedeacidifying section B. The amount of water added to the fluidizing gasfor this purpose is preferably at least 0.1 vol %, and most preferably0.1 to 1 vol %. In the absence of moisture, the silica may become lessflowable, making it necessary to use more fluidizing gas, which in turnresults in increased fly-out. This is particularly undesirable from thestandpoint of the burden on the bag filter. On the other hand, too muchmoisture may give rise to such undesirable effects as condensation whenthe deacidified silica is recovered in a recovery vessel 24 from thedeacidifying section B.

[0028] The deacidification temperature is preferably 400 to 500° C., andthe flow velocity is preferably 1 to 6 cm/s.

[0029] Waste gases from the fluidization vessel 9 (including bothhydrophobizing section A and deacidifying section B) are sent to ascrubber via a cyclone 18 and a bag filter 19. Silica accompanying thewaste gases passes from the cyclone 18 to a rotary valve 20 or istrapped by the bag filter 19, then is collected in a hopper 21,following which it is returned to the deacidifying section B via arotary valve 22 and a diaphragm pump 23. The deacidified silica iscollected in the recovery vessel 24.

[0030] The silica that flew out of the fluidization vessel 9 togetherwith the waste gases was collected and the physical properties examined.Treatment appeared sufficient in terms of the carbon content, but the pHwas 3.7 to 4.1, indicating a need to again deacidify the collectedsilica. Hence, the silica collected by the cyclone 18 and bag filter 19are fed by a diaphragm pump 23 to the center of the deacidifying sectionB of the fluidization vessel 9. Unreacted silane accompanies the wastegases. The condensation of moisture accompanying the waste gases on thewalls of the apparatus at temperatures below 100° C. converts the silaneinto a gel or oil, which obstructs pipelines and in particular clogs thepores of the filter fabric used in the bag filter 19. Accordingly, it isnecessary to maintain the interior of the system at a temperature of atleast 100° C. In FIG. 1, T1 and T2 are each thermometers which measurethe temperature of the waste gases. The temperature readings at T1 andT2 must be at least 100° C., although a higher temperature, such as 130°C. or more, is preferred at the bag filter, both for the gasesthemselves and also for areas of the bag filter that come into directcontact with the gases. Accordingly, the interior of the exhaust systemmust be held at a temperature within a range of 100 to 500° C. and, forreasons associated in part with the choice of filter fabric and bagfilter, preferably in a range of 130 to 200° C. The formation of gummyor oily deposits on the filter fabric of the bag filter 19 causes thepressure difference to rise, making normal operation difficult. It isthus desirable to install a differential pressure gauge P1 on the bagfilter 19 to monitor changes in the pressure difference. The productionapparatus shown in FIG. 1 is also provided with a heat insulator 28 anda steam tracer 29 to keep the temperature from falling.

[0031] The properties of the hydrophobic silica produced by thetreatment method and apparatus of the invention are not subject to anyparticular limitation, although a specific surface area of about 110m²/g, a carbon content of at least about 0.9 wt %, and a pH of at least4.5 are preferred. Hydrophobic silica having such properties is highlysuitable for use in sealants and related applications.

EXAMPLE

[0032] The following examples are provided to illustrate the invention,and are not intended to limit the scope thereof.

Example 1

[0033] The apparatus shown in FIG. 1 was operated continuously for atotal of 500 hours. During operation, 50.3 kg/h of methyltrichlorosilanewas burned together with hydrogen and air, producing 20.1 kg/h ofsilica. The resulting silica was subjected to hydrophobizing treatmentat a nitrogen feed rate of 30 Nm³/h, a dimethyldichlorosilane feed rateof 2.0 kg/h, and a water feed rate of 0.5 kg/h into section A of thefluidization vessel 9, and a temperature of 490° C. The flow velocity ofsilica into section A was 2.0 cm/s. The hydrophobized silica was thendeacidified at a nitrogen feed rate of 35 Nm³/h and a water feed rate of0.2 kg/h to section B of the fluidization vessel 9, a temperature of480° C., and a flow velocity of about 2.2 cm/s. The treated silica had,on average, a specific surface area of 114 m²/g, a carbon content of0.97 wt %, and a pH of 4.7. The temperatures of the cyclone 18 and thebag filter 19 were, on average, 150° C. (T1) and 135° C. (T2). Thepressure difference P1 at the bag filter was 0.8 kPa at the start ofoperation, and 1.4 kPa at the end of operation. The combined amount ofsilica collected by the cyclone 18 and the bag filter 19 on thedischarge side of the diaphragm pump 23 during operation was 0.8 kg/h,representing a fly-out ratio of about 4%. Following the end ofoperation, the scrubber fluid was almost entirely free of suspendedsilica. Nor was there any gel or oil deposited on the filter fabric ofthe bag filter.

[0034] In another run, using the apparatus shown in FIG. 1,methyltrichlorosilane was burned to form 20 kg/h of silica, and thesilica was treated for 6 hours with dimethyldichlorosilane, whereupon anaverage of 1.4 kg/h of fugitive silica was recovered at the diaphragmpump 23 outlet. In a further run wherein dimethyldichlorosilane was notsupplied and only deacidification was carried out, the amount offugitive silica recovered was 0.07 kg/h. Each of the above runs wascarried out several times, whereupon the fly-out ratio was 0.3 to 0.5%without hydrophobization, and increased considerably to 4 to 15% withhydrophobization.

Example 2

[0035] The apparatus shown in FIG. 1 was operated for a period of 7hours by burning 49.6 kg/h of methyltrichlorosilane together withhydrogen and air, thereby producing 19.8 kg/h of silica.Hydrophobization of the silica was carried out in section A of thefluidization vessel 9 in the same manner as in Example 1.Deacidification was carried out in section B of the fluidization vesselB without feeding water and at a nitrogen feed rate of 45 Nm³/h, atemperature of 480° C., and a flow velocity of about 2.8 cm/s. Thetreated silica had a specific surface area of 114 m²/g, a carbon contentof 0.95 wt %, and a pH of 4.6. The amount of silica collected on thedischarge side of the diaphragm pump 23 was 2.4 kg/h. Hence, the fly-outratio was about 12%.

Example 3

[0036] The apparatus shown in FIG. 1 was operated for a period of 7hours by burning 50.4 kg/h of methyltrichlorosilane with hydrogen andair, thereby producing 20.1 kg/h of silica. Treatment in section A wasthe same as in Example 1. Aside from feeding 2.0 kg/h of water tosection B, treatment in section B was also carried out as in Example 1.The treated silica had a specific surface area of 108 m²/g, a carboncontent of 0.95 wt %, and a pH of 4.8. The amount of silica collected onthe discharge side of the diaphragm pump 23 was 1.2 kg/h, indicating afly-out ratio of about 6%. Condensation was observed in the silicarecovery vessel 24 on the outlet side of the fluidization vessel 9.

Comparative Example 1

[0037] The apparatus shown in FIG. 2 was used. Referring to FIG. 2, theapparatus included fluidization vessels 31 and 32, a hydrophobizingagent container 33, a constant-temperature vessel 34, water tanks 35 and37, pumps 36 and 38, a bag filter 39, a heater 40 and a heat insulator42. Other parts serving the same purposes as parts in FIG. 1 aredesignated by the same reference numerals. The apparatus also includesthermometers T51, T52 and T53, and a differential pressure gauge P51.

[0038] About 3 kg/h of methyltrichlorosilane was burned with hydrogenand air, yielding about 1.2 kg/h of silica. Hydrophobization was carriedout at a temperature of 500° C. in fluidization vessels 31 and 32, adimethyldichlorosilane feed rate of 0.12 kg/h, a water feed rate to thefluidization vessel 31 of 0.03 kg/h, and a water feed rate to thefluidization vessel 32 of 1 part by volume per 100 parts by volume ofthe fluidizing gas. The flow velocity was about 2.7 cm/s. The treatedsilica had a specific surface area of 115 m²/g, a carbon content of 0.92wt %, and a pH of 4.5. The average temperatures were 110° C. at T51, 90°C. at T52, and 75° C. at T53. The reading on the differential pressuregauge P51 at the bag filter was 0.7 kPa at the start of operation.However, this rose to 2.8 kPa, and so operation was stopped after atotal of 40 hours. After the end of operation, the filter fabric at thebag filter had an oily and tacky feel. No oil or gel deposits were foundon the walls of the pipeline at T51, but considerable deposits werenoted at T52.

[0039] As demonstrated in the foregoing examples, the inventive methodand apparatus use a cyclone and a bag filter to recover silica thatflies out of the fluidization vessel. Under simple controlled conditionsthat involve holding these devices at temperatures of at least 100° C.,essentially 100% of the fugitive silica can be recovered, resulting inincreased yield of the product and reducing the burden on waste gastreatment.

[0040] Japanese Patent Application No. 2000-262219 is incorporatedherein by reference.

[0041] Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. An apparatus for producing hydrophobic silica fine powder,comprising: a means for pyrolyzing a silane compound to form silica finepowder, a means for agglomerating the silica fine powder, a firstcyclone and a first filter for collecting the agglomerated silica finepowder, a fluidization vessel having a hydrophobizing section forhydrophobizing the collected silica fine power, and a second cyclone anda second filter for collecting hydrophobic silica fine powder whichflies out of the fluidization vessel, which second cyclone and secondfilter can each be held at a temperature of 100 to 500° C.